1. Field of the Invention
The present invention relates to the preparation of flakes of hydrophilic (water-swellable) or water-soluble acrylic polymers of polyacrylamide type, and also to the discrete, non-adherent, free-flowing flakes, per se, thus prepared.
2. Description of the Prior Art
Water-soluble acrylic homopolymers and copolymers having a high molecular weight are adapted for many useful applications in the form of dilute aqueous solutions thereof, more particularly as flocculating agents used in waste water processing, or as thickening agents in the paper and petroleum industries. Hydrophilic acrylic copolymers are characterized as materials which are not water-soluble but which absorb aqueous fluids; these copolymers may more particularly be employed as water-retaining agents in the fields of agriculture, horticulture, floriculture, etc.
These polymers may be obtained by polymerization of acrylic monomers in aqueous solution. Through this type of process, the polymer is provided in the form of sheets, layers or blocks of a solid gel having a plastic, rubbery consistency and which must subsequently be chopped, shredded or otherwise comminuted using a special device of the extruder or "meat grinder" type.
It is not possible to directly package the polymer in the form of a chopped or shredded gel, because the rubbery, adhesive pieces agglomerate and become fused during storage. The gels are therefore normally dehydrated, dried and ground into a fine powder. However, drying and grinding are processes that are expensive, as they entail a substantial investment and high operating expenses and they also can result in deterioration of the polymer. In the case of the water-soluble polymers, such deterioration results in reduced viscosity in solution and/or the formation of insoluble microgels unless adequate precautions are taken. In addition, the dried and ground powder contains fine, dust-generating particles that tend to form flocs that dissolve with difficulty during preparation of aqueous solutions therefrom.
Various methods for processing polyacrylamide powders or gels have to date been proposed to this art for avoiding agglomeration, reducing the formation of dust particles, or improving the fluidity and/or increasing the speed of dissolution thereof in water.
Thus, according to U.S. Pat. No. 4,080,358, polyacrylamides in a finely divided state are mixed with a poly(alkylene)glycol and an anti-clotting agent (free-flow aid) such as a silica or silicate. The anti-clotting agent is added after drying the gel and sifting to the appropriate size. According to French Pat. No. 2,368,503, the problems of the adhesive properties of the surface of a hydrogel of the polyacrylamide type are remedied by coating same with a fatty acid or an alkaline salt thereof. The fatty acid is used in a proportion of approximately 0.01% and can be added at the polymerization stage or by sprinkling or pulverizing same over the gel, either prior to grinding or after grinding. It is specified that the result obtained is substantially the same regardless of the method employed. After grinding, the gel particles are dried.
According to another known process (French Pat. No. 2,150,973), polyacrylamide gels in the stable form, that are not adhesive and that contain 40 to 90% water are ground and subsequently sprinkled with an inorganic or organic substance such as a starch, a dextrin, a silicate, a clay, or activated silica.
However, through these processes non-adhesive, very small granules are not directly produced by the device in which the gel is ground.